Constant velocity or CV joints, as used on automotive drive axles, such as on front wheel drive cars, are commonly enclosed within a convoluted polymer boot. Such convoluted boots, that is, boots having a plurality of axially spaced annular convolutions, are commonly formed with a small end which is retained by a clamp on the drive shaft leading out of the joint, and a large end clamped to an annular surface of the body of the CV joint. Such boots provide a seal for the joint over the operating range of the constant velocity joint.
Convoluted boots may be blow molded of rigid thermoplastic polyester elastomers by reason of their mechanical and physical strength over a wide variety of operating conditions, and generally by reason of their toughness and their ability to resist puncture or tearing. However, such blow molded elastomeric polymers are somewhat hard and stiff and can require a substantially large compressive clamping force to hold the large end of the boot in place on the body of the CV joint, without movement under the clamp, while still providing an effective seal between the boot and the body.
Typical materials from which blow-molded convoluted boots have been made are commonly referred to as thermoplastic elastomers (TPE) and include polyetherpolybutylterephalate compounds (PEPBT). Typical thermoplastic elastomer materials which are used for blow-molding convoluted boots, as defined above, include E. I. du Pont de Nemours and Company "Hytrel", HTG-5612 and Monsanto's "Santoprene" thermoplastic rubber, typically grade 103-40. Such elastomers, as compared to typical rubbers, have a substantially greater tendency to deform at relatively low elongation forces, and typically have a maximum or 100% modulus which is less than half of the ultimate tensile stress.
The relatively high stiffness of such blow-molded elastomers normally suggests the employment of a correspondingly heavier clamp with high clamping forces in order to hold the boot in place on the body, particularly when the boot is operated under angular offset conditions, such as on a constant velocity joint. This is especially the case at the large end of the boot, where the large inside diameter is clamped to the joint housing. In this instance, a combination of dissimilarities combine to make more difficult the clamping of such a convoluted boot, particularly at its larger end. These include (a) the large force required to compress TPE material, by reason of its substantial hardness as mentioned above; (b) the ability of the blow-molded TPE material to transmit axial loads onto the clamp; (c) the relatively inability of such TPE material to accept any high amount of tensile loads without permanent deformation; and (d) the tendency of the TPE blow-molded polymer material to cold flow under the clamp. For example, the compression set of E. I. du Pont de Nemours and Company "Hytrel" tested in accordance with ASTM-D395 method A, at 100.degree. C. is 8%. However, rubber elastomers are normally tested under ASTM-D395 method B, which measures compression set under constant deflection. Such 8% compression set occurs at only about 9% strain and this would be translated as almost a 100% compression set by ASTM-D395 method B. This explains why, in service, most of the problems of sealing blow-molded TPE convoluted boots is that of the replacement of a clamp which has come loose.
A particular problem in sealing constant velocity joints with blow-molded boots resides in conforming a boot to the unusual configuration of closed tripod joints, often referred to a trilobal-tripot joints, of the kind shown in Sutton et al, U. S. Pat. No. 4,795,404 issued Jan. 3, 1989. The outer housing of this joint is not circular, but is formed with three equally spaced lobes. Typically the convoluted elastomer boot is attached to a trilobal-tripot constant velocity (TTCV) joint housing by placing a metallic can over an elastomeric bushing to achieve a cylindrical shape and by using a clamp to seal and secure the convoluted elastomer boot around the can. This configuration, while allowing the convoluted boot to be smaller in diameter at the end of the housing to minimize the amount of lubricating grease required, is expensive to manufacture and is therefore undesirable.
In another trilobal-tripot joint assembly as shown in U.S. Pat. No. 4,795,404, an elastomeric or thermoplastic elastomer (TPE) filler ring is placed between the joint housing and the convoluted boot. This assembly while less expensive than using a metallic can, requires a large housing end on the convoluted boot and a greater amount of lubricating grease. Due to the necessity of making the filler ring pliable enough to seal in a compression type of load and the convoluted boot strong enough to withstand puncture, the clamp is undesirably forced to compress a more rigid material tube down on a more flexible material, in those cases where the boot is formed of PEPBT or similar polymers.